Kofron et al U.S. Pat. No. 4,439,520 ushered in the current era of high performance silver halide photography. Kofron et al discloses chemically and spectrally sensitized high aspect ratio tabular grain emulsions in which tabular grains having a diameter of at least 0.6 .mu.m and a thickness of less than 0.3 .mu.m exhibit an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. Kofron et al in column 11, lines 55 to 58 inclusive, states that the tabular grains typically have a thickness of at least 0.03 .mu.m, but can in theory have thicknesses as low as 0.01 .mu.m. Kofron et al in column 89, Table XVIII reports a series of tabular grain silver bromide emulsions having tabular grain thicknesses ranging from 0.07 to 0.12 .mu.m and projected areas of greater than 95 percent of total grain projected area; however, in column 94, Table XXI a parallel preparation of tabular grain silver bromoiodide emulsions shows tabular grain thicknesses ranging from 0.08 to 0.11 .mu.m, showing some thickening of the grains, and tabular grain projected areas as a percentage of total grain projected area are sharply reduced to just greater than 85 percent of total grain projected area. In column 15, line 50, Kofron et al states that emulsions having coefficients of variation of less than 30 percent can be prepared, but from FIG. 3 (showing a wide grain dispersity) and the numerous Example emulsions having tabular grain projected areas in the range of from just greater than 50 to just greater than 70 percent, it is apparent that for the most part the emulsions did not have coefficients of variation of less than 30 percent.
Kofron et al recognized that the tabular grain emulsions would produce both single and multiple emulsion layer photographic elements exhibiting improved photographic performance in terms of image structure (sharpness and granularity) and enhanced photographic speed as a function of image structure--e.g., an improved speed-granularity relationship. A series of multicolor photographic element layer order arrangements containing a high aspect ratio tabular grain emulsion in one or more layers is disclosed by Kofron et al in columns 56 to 58. In column 79, Table XII comparisons are provided of green and red image sharpness within multicolor photographic elements containing fast and slow blue light recording (yellow image dye forming), green light recording (magenta image dye forming) and red light recording (cyan image dye forming) emulsion layers containing various selections of nontabular grain emulsions set out in column 28, Table X, and tabular grain emulsions set out in column 28, Table XI. Note that while the tabular grain emulsions ranged from 0.06 to 0.19 .mu.m in thickness, the percentage of tabular grain projected area did not range appreciably above 70 percent of total grain projected area.
A preferred technique employed by Kofron et al for the preparation of the high aspect ratio tabular grain silver bromide and bromoiodide emulsions is disclosed starting at column 13, line 15, and extending through column 16, line 48. Grain nucleation is preferably undertaken by the double jet precipitation of silver bromide grain nuclei that are substantially free of iodide in the pBr range of from 0.6 (preferably 1.1) to 1.6 (preferably 1.5). It is stated (col. 14, lines 15 to 19) that if the pBr of the dispersing medium is initially too high, the tabular grains will be comparatively thick. In the first paragraph of column 15 it is stated that instead of introducing silver, bromide and iodide as aqueous solutions initially or during the growth stage it is alternatively possible to introduce fine silver halide grains--e.g. grains having a mean diameter of less than 0.1 .mu.m.
Kofron et al (col. 13, lines 42-50) suggests ultrafiltration during precipitation, as taught by Mignot U.S. Pat. No. 4,334,012. Mignot teaches a general process for the ultrafiltration of silver halide emulsions during precipitation that is equally applicable to tabular and nontabular grain emulsion precipitations. In its simplest form Mignot contemplates the nucleation and growth stages of silver halide precipitation occurring in the same reaction vessel. In column 14, line 21, through column 15, line 16, it is suggested to perform grain nucleation and growth in separate reaction vessels. Return of emulsion from the ultrafiltration unit to either the nucleation or growth reaction vessels is contemplated. Urabe U.S. Pat. No. 4,879,208, Verhille et al U.S. Pat. No. 4,171,224 and Forster et al U.S. Pat. No. 3,897,935, disclose grain nucleation upstream of a growth reaction vessel.
Several hundred scientific and patent publications have followed Kofron et al purporting to represent alternatives in terms of one or more tabular grain emulsion parameters and/or variations of processes for tabular grain emulsion preparation. Attention is specifically directed to the following:
Daubendiek et al U.S. Pat. No. 4,414,310 discloses high aspect ratio tabular grain emulsions prepared using silver iodide seed grains. Average tabular grain thicknesses as low as 0.06 .mu.m are disclosed with tabular grain projected areas of just greater than 90 percent of total grain projected area. A high proportion of the tabular grains have hexagonal major faces.
Research Disclosure, Aug. 1983, Item 23212, discloses a process of preparing silver bromide high aspect ratio tabular grain emulsions in which the tabular grains account for at least 97 percent of total grain projected area and have an average thickness of at least 0.03 .mu.m. In Example 1 at least 99 percent of the total grain projected area is accounted for by silver bromide tabular grains having an average thickness of 0.06 .mu.m. The coefficient of variation of the emulsion is 15. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England. The tabular grains are prepared by a double jet precipitation to form seed grains followed by ripening in the absence of a nonsilver halide solvent. Ultrafiltration while forming the seed grains as taught by Mignot, cited above, is specifically taught.
Abbott et al U.S. Pat. No. 4,425,426 discloses thin, intermediate aspect ratio tabular grain emulsions in which tabular grains having thicknesses of less than 0.2 .mu.m have average aspect ratios in the range of from 5 to 8. Tabular Grain Emulsion 1 exhibited an average tabular grain thickness of 0.09 .mu.m with tabular grains accounting for just greater than 75 percent of total grain projected area.
Daubendiek et al U.S. Pat. No. 4,693,964 discloses that increased image sharpness can be achieved in an underlying minus blue recording silver halide emulsion layer of a multicolor photographic element when an overlying tabular grain emulsion layer is provided in which at least 50 percent of total grain projected area is accounted for by tabular grains having an average aspect ratio of greater than 8 and an average equivalent circular diameter of from 0.4 to 0.55 .mu.m. A series of tabular grain emulsions are listed in Table I, column 22. From comparisons presented in the Examples it is taught that increasing the average equivalent circular diameter of the tabular grains in the overlying emulsion layer to a value of 0.64 .mu.m, as illustrated by emulsion TC17, results in obtaining inferior image sharpness in the underlying emulsion layer. Thus, the teaching of Daubendiek et al is that a sharpness penalty is incurred in an underlying minus blue sensitized emulsion layer when the tabular grains in an overlying emulsion layer have an average equivalent circular diameter that exceed 0.55 .mu.m. A remake of emulsion TC17 of Daubendiek et al appears in the Examples below as Control Emulsion TC12.
Maskasky U.S. Pat. No. 4,713,320 discloses that the proportion of unwanted grain shapes (principally rods) in tabular grain silver bromide or bromoiodide emulsions can be reduced by employing during precipitation a gelatino-peptizer containing less than 30 micromoles of methionine per gram. In column 14, Emulsion 8B, a silver bromoiodide emulsion is reported prepared in the presence of low methionine gelatin in which tabular grains having a mean diameter of 2.6 .mu.m and a mean thickness of 0.071 .mu.m account for more than 85 percent of total grain projected area.
Saitou et al U.S. Pat. No. 4,797,354 reports tabular grain emulsions in which a high proportion of the tabular grains have hexagonal major faces with a 2:1 or less ratio of adjacent edge lengths. Low coefficients of variation of the tabular grains are reported (not to be confused with customary and significantly higher coefficient of variation measurements based on emulsion total grain population). Although silver halide emulsions of varied halide compositions are disclosed, only silver bromide emulsions are reported in the Examples.
Zola and Bryant published European patent application 362699 A3 discloses silver bromoiodide tabular grain emulsions of reduced dispersity in which the average aspect ratio of the silver bromoiodide tabular grains divided by the coefficient of variation of the total silver bromoiodide grain population is greater than 0.7. Examples 5 to 7 inclusive disclose tabular grain silver bromoiodide emulsions in the average tabular grain thickness is less than 0.07 .mu.m, with the lowest coefficient of variation reported for these emulsions being 38 percent. In Example 3 the tabular grains exhibited an average thickness of 0.12 and accounted for 88 percent of the total grain projected area, with the coefficient of variation of the total grain population being 23 percent.